The present invention relates to a novel method for determining refractive index, and to the use of this method in determining qualitatively or quantitatively the presence of a substance in a gaseous, liquid or solid sample. The invention also relates to a sensor device for carrying out the method.
In its simplest form, a ring resonator is comprised of an open (normally straight) optical waveguide which is located closely adjacent to a closed (normally annular or ring-shaped) optical waveguide. When light from a single-mode laser, i.e. a laser which transmits laser light of single wavelength, is coupled into the open waveguide, some of this light will also be coupled to the annular waveguide in the region where the two waveguides lie close together. Normally, the light concerned in this regard will constitute only one or some percent of the total light. However, if the wavelength of the laser light is such that the waveguide ring is able to accommodate precisely a whole number of wavelengths, the resonance conditions of the ring for this wavelength are fulfilled and substantially more light will be coupled to the ring and be stored therein, which, in turn, results in a corresponding reduction in the amount of light that is transmitted at the output end of the open waveguide. Thus, when seen over a range of wavelengths, the resonance will be observed at the output end of the open waveguide as a sharp minimum in the power of the transmitted light. This phenomenon has been utilized in rotary sensors, such as the gyro, among other things, and also to determine very low propagation losses in waveguides.
The resonance condition for a ring resonator can be written as: EQU 2.multidot..pi..multidot.R.multidot.n.sub.eff =m.multidot..lambda.
where R is the radius of the ring, n.sub.eff is the effective refractive index of the waveguide, m is an integer and .lambda. is the wavelength of the laser light. In order to be able to detect a resonance, it is necessary to vary the laser light wavelength .lambda. and to detect the effect upon the light coming from the open waveguide. It will be seen from the formula that when the effective refractive index n.sub.eff of the ring changes, the resonance wavelengths will also change. The effective refractive index (n.sub.eff) is, in turn, influenced by all media in which the wave-guided light propagates, mostly in the actual core of the waveguide although also in materials closely surrounding the core.
WO 91/03728 describes the use of a waveguide based sensor system for determining the concentration of a chemical substance, wherein there is produced two mutually separate evanescent fields having mutually different penetration depths. According to one embodiment, the system includes a ring resonator and there is determined a relative absorption change in relation to the concentration of the chemical substance.